Joint strengthening ring for graphite electrodes

ABSTRACT

An electrode joint is presented, the joint including two joined graphite electrodes and having a joint strengthening ring interposed between the electrodes, the joint strengthening ring composed of a compressible.

RELATED APPLICATION

This application is a continuation in part of and commonly assigned U.S.patent application Ser. No. 10/760,947, filed on Jan. 20, 2004 nowabandoned in the names of Bowman, Wells, Weber and Pavlisin, entitled“End-Face Locking Ring for Graphite Electrodes,” the disclosure of whichis incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a joint strengthening ring for graphiteelectrodes, and a process for preparing the inventive strengtheningring. More particularly, the invention concerns a ring, advantageouslyformed of particles of expanded graphite, used at the end faces ofgraphite electrodes formed in a joint to improve the high column bendingstrength of an electrode column of which the inventive ring and joint isa member. By “high column bending strength” is meant the ability of agraphite electrode column to resist cracks, splits or other deleteriouseffects brought about by the bending forces to which the column issubjected during the operation of an electric arc furnace (“EAF”).

2. Background Art

Graphite electrodes are used in the steel industry to melt the metalsand other ingredients used to form steel in electrothermal furnaces. Theheat needed to melt metals is generated by passing current through aplurality of electrodes, usually three, and forming an arc between theelectrodes and the metal. Electrical currents in excess of 100,000amperes are often used. The resulting high temperature melts the metalsand other ingredients. Generally, the electrodes used in steel furnaceseach consist of electrode columns, that is, a series of individualelectrodes joined to form a single column. In this way, as electrodesare depleted during the thermal process, replacement electrodes can bejoined to the column to maintain the length of the column extending intothe furnace.

Generally, electrodes are joined into columns via a pin (sometimesreferred to as a nipple) that functions to join the ends of adjoiningelectrodes. Typically, the pin takes the form of opposed male threadedsections, with at least one end of the electrodes comprising femalethreaded sections capable of mating with the male threaded section ofthe pin. Thus, when each of the opposing male threaded sections of a pinare threaded into female threaded sections in the ends of twoelectrodes, those electrodes become joined into an electrode column.Commonly, the joined ends of the adjoining electrodes, and the pintherebetween, are referred to in the art as a joint.

Alternatively, the electrodes can be formed with a male threadedprotrusion or tang machined into one end and a female threaded socketmachined into the other end, such that the electrodes can be joined bythreading the male tang of one electrode into the female socket of asecond electrode, and thus form an electrode column. The joined ends oftwo adjoining electrodes in such an embodiment is also referred to inthe art as a joint.

Given the extreme thermal stress that the electrode and the joint (andindeed the electrode column as a whole) undergoes, mechanical/thermalfactors such as strength, thermal expansion, and crack resistance mustbe carefully balanced to avoid damage or destruction of the electrodecolumn or individual electrodes. For instance, longitudinal (i.e., alongthe length of the electrode/electrode column) thermal expansion of theelectrodes, especially at a rate different than that of the pin, canforce the joint apart, reducing effectiveness of the electrode column inconducting the electrical current. A certain amount of transverse (i.e.,across the diameter of the electrode/electrode column) thermal expansionof the electrode in excess of that of the pin may be desirable to form afirm connection between pin and electrode; however, if the transversethermal expansion of the electrode greatly exceeds that of the pin,damage to the electrode or separation of the joint may result. Again,this can result in reduced effectiveness of the electrode column, oreven destruction of the column if the damage is so severe that theelectrode column fails at the joint section.

Moreover, another effect of the thermal and mechanical stresses to whichan electrode column is exposed is damage to the electrode making up thecolumn due to “bending” forces applied to the column. This can result incracks or splits in one or more of the electrodes, or other deleteriouseffects. These conditions can reduce electrode column efficiency byreducing electrical contact between adjoining electrodes. In the mostsevere case, cracks and splits can result in breakage, with resultingloss of the electrode column below the affected electrode.

In U.S. Pat. No. 3,540,764, Paus and Revilock suggest the use of anexpanded graphite spacer disposed between the end faces of adjacentelectrodes in order to increase electrical conductivity and thermalstress resistance of the joint. The nature of the Paus and Revilockspacer and its placement, however, is such that a gap is created in thejoint where it may not have otherwise been, thereby contributing tojoint looseness and potential for failure.

What is desired, therefore, is a joint strengthening ring that can beused to reduce the tendency of electrodes in a column to crack, split orotherwise be damaged by bending forces to which the column is subjectedduring the operation of the EAF. In other words, the desired electrodejoint strengthening ring increases the high column bending strength ofthe column. It is also highly desirable to achieve these propertybenefits without using high quantities of expensive materials withoutrequiring a substantial amount of effort at the electric arc furnacesite.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a jointstrengthening ring for the end faces of graphite electrodes.

It is another aspect of the present invention to provide a jointstrengthening ring for the end faces of graphite electrodes whichimproves the high column bending strength of an electrode column formedfrom such electrodes.

It is yet another aspect of the present invention to provide a jointstrengthening ring for the end faces of graphite electrodes whichproduces electrode column joints having improved strength and stability.

Still another aspect of the present invention is a graphite electrodejoint, having improved resistance to damage caused by bending forces ascompared to art-conventional graphite electrode joints.

These aspects and others that will become apparent to the artisan uponreview of the following description can be accomplished by providing anelectrode joint comprising two joined graphite electrodes and having ajoint strengthening ring interposed between the electrodes, the jointstrengthening ring comprising a compressible material, especiallycompressed particles of exfoliated graphite. In a preferred embodiment,the electrical conductivity of the joint strengthening ring is greaterin the direction extending between the electrodes than it is in thedirection orthogonal thereto. In order to accomplish this, the jointstrengthening ring should advantageously comprise a spiral wound sheetof compressed particles of exfoliated graphite.

The two joined electrodes forming the joint can each comprise a femalethreaded socket machined therein and further comprising a pin comprisingopposed male threaded sections which engage the female threaded socketsof the electrodes to form the joint. Alternatively, one of theelectrodes can comprise a male threaded tang and the other electrode cancomprise a female threaded socket, wherein the male threaded tangengages the female threaded socket to form the joint.

Preferably, to form the inventive joint strengthening ring, a sheet ofcompressed particles of exfoliated graphite is provided and then wound(for instance around a bolster having a diameter equal to the inneropening of the joint strengthening ring) to form a spiral wound jointstrengthening ring suitable for use between the electrodes in anelectrode joint. The joint strengthening ring should have an outerdiameter generally equal to the outer diameter of the electrode jointand an inner opening, and can but does not necessarily have an adhesiveinterposed between the layers of the spiral wound sheet of compressedparticles of exfoliated graphite.

In addition to being formed of a compressible material such as spiralwound sheets of compressed particles of exfoliated graphite, theinventive joint strengthening ring can be shaped so as to increase itscompressibility, such as by molding. For example, the sheet can bemolded so as to assume a concave shape when viewed along the plane ofthe end faces of one or both of the electrodes between which the jointstrengthening ring is situated. The space between the tapered “arms” ateither end of the concavity provides even greater potential forcompressibility. Moreover, a ramming paste, cement or other putty-likematerial can be positioned in the concave space. Another waycompressibility of the spiral wound exfoliated graphite sheets can beincreased is by forming a “rippled” or “corrugated” surface of the jointstrengthening ring, also by molding. The concave or corrugated surfacesof the joint strengthening ring are, of course, one or both of thesurfaces which abut the respective electrode end faces.

It is to be understood that both the foregoing general description andthe following detailed description provide embodiments of the inventionand are intended to provide an overview or framework of understandingand nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention and are incorporated in and constitute a part of thespecification. The drawings illustrate various embodiments of theinvention and together with the description serve to describe theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side perspective view of an end-face joint strengtheningring for a graphite electrode in accordance with the present invention.

FIG. 2 is a side perspective view of a spiral wound flexible graphitestructure from which the end-face joint strengthening ring of FIG. 1 isderived.

FIG. 3 is a partial side perspective view of a male threaded graphiteelectrode having an end-face joint strengthening ring in accordance withthe present invention thereon.

FIG. 4 is a partial side perspective view of a graphite electrode havinga pin threaded thereinto and having an end-face joint strengthening ringin accordance with the present invention thereon.

FIG. 5 is a side plan view of an electrode joint having an end-facejoint strengthening ring in accordance with the present inventiontherein.

FIG. 6 is a side cross-sectional view of one embodiment of an end-facejoint strengthening ring for graphite electrodes in accordance with thepresent invention.

FIG. 7 is a side cross-sectional view of another embodiment of anend-face joint strengthening ring for graphite electrodes in accordancewith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Graphite electrodes can be fabricated by first combining a particulatefraction comprising calcined coke, pitch and, optionally, mesophasepitch or PAN-based carbon fibers into a stock blend. More specifically,crushed, sized and milled calcined petroleum coke is mixed with acoal-tar pitch binder to form the blend. The particle size of thecalcined coke is selected according to the end use of the article, andis within the skill in the art. Generally, in graphite electrodes foruse in processing steel, particles up to about 25 millimeters (mm) inaverage diameter are employed in the blend. The particulate fractionpreferable includes a small particle size filler comprising coke powder.Other additives that may be incorporated into the small particle sizefiller include iron oxides to inhibit puffing (caused by release ofsulfur from its bond with carbon inside the coke particles), coke powderand oils or other lubricants to facilitate extrusion of the blend.

After the blend of particulate fraction, pitch binder, etc. is prepared,the body is formed (or shaped) by extrusion though a die or molded inconventional forming molds to form what is referred to as a green stock.The forming, whether through extrusion or molding, is conducted at atemperature close to the softening point of the pitch, usually about100° C. or higher. The die or mold can form the article in substantiallyfinal form and size, although machining of the finished article isusually needed, at the very least to provide structure such as threads.The size of the green stock can vary; for electrodes the diameter canvary between about 220 mm and 700 mm.

After extrusion, the green stock is heat treated by baking at atemperature of between about 700° C. and about 1100° C., more preferablybetween about 800° C. and about 1000° C., to carbonize the pitch binderto solid pitch coke, to give the article permanency of form, highmechanical strength, good thermal conductivity, and comparatively lowelectrical resistance, and thus form a carbonized stock. The green stockis baked in the relative absence of air to avoid oxidation. Bakingshould be carried out at a rate of about 1° C. to about 5° C. rise perhour to the final temperature. After baking, the carbonized stock may beimpregnated one or more times with coal tar or petroleum pitch, or othertypes of pitches or resins known in the industry, to deposit additionalcoke in any open pores of the stock. Each impregnation is then followedby an additional baking step.

After baking, the carbonized stock is then graphitized. Graphitizationis by heat treatment at a final temperature of between about 2500° C. toabout 3400° C. for a time sufficient to cause the carbon atoms in thecoke and pitch coke binder to transform from a poorly ordered state intothe crystalline structure of graphite. Advantageously, graphitization isperformed by maintaining the carbonized stock at a temperature of atleast about 2700° C., and more advantageously at a temperature ofbetween about 2700° C. and about 3200° C. At these high temperatures,elements other than carbon are volatilized and escape as vapors. Thetime required for maintenance at the graphitization temperature usingthe process of the present invention is no more than about 18 hours,indeed, no more than about 12 hours. Preferably, graphitization is forabout 1.5 to about 8 hours. Once graphitization is completed, thefinished article can be cut to size and then machined or otherwiseformed into its final configuration.

The inventive joint strengthening ring comprises a material that isdisposed in an electrode joint between the end-faces of the adjoiningelectrodes. The joint strengthening ring preferably comprises a materialsized so as to fill the gap between the adjoining electrodes. To thatend, the joint strengthening ring should advantageously be between about1 mm and about 25 mm in thickness, more advantageously, between about 3mm and about 12 mm in thickness. In addition, the joint strengtheningring should extend radially from the perimeter of the electrode joint intowards the center of the joint, terminating between the perimeter andthe threaded pin or male threaded tang. Most preferably, the radius ofthe joint strengthening ring should be approximately equal to that ofthe electrodes between which it is disposed. Thus, the inventive jointstrengthening ring should have a radius of between about 11 cm and about37 cm (when used with graphite electrodes having a circularcross-section), more preferably between about 20 cm and about 30 cm,with a central opening having a diameter approximately equal to orlarger than the diameter of the threaded pin or male tang (at theirrespective largest point); more particularly, the diameter of thecentral opening of the joint strengthening ring should be between about50% and about 85% of the diameter of the electrodes between which it isdisposed. In the most preferred embodiment, the central opening of thejoint strengthening ring should be approximately equal to the diameterof threaded pin or male tang (at their respective largest point).

The material(s) from which the inventive joint strengthening ring isproduced or the orientation or placement of the joint strengtheningring, should be such that the joint strengthening ring is compressibleto compensate for differences and changes in the gap between adjoiningelectrodes, which can vary based on the method used to connect theadjoining electrodes, as well as due to the different mechanical andthermal stresses to which the joint is exposed while in use in thefurnace. In addition, compressibility of the joint strengthening ringmaterial can help ensure that air does not pass between the jointstrengthening ring and the electrodes between which it is positioned.

The material from which the joint strengthening ring of the presentinvention is formed can also advantageously function to slow the rate atwhich the threads of the electrode joint oxidize. To do so, it has toreduce (or physically block) the exposure of the threads to the hot airin the furnace. More preferably, the joint strengthening ring materialshould oxidize at a rate equal to or slower than that of the electrodesforming the joint. Most preferably, the joint strengthening ringmaterial should oxidize at as slow a rate as possible, while meeting thecompressibility requirements.

Suitable materials useful for forming the inventive joint strengtheningring include paper, cardboard, paste, braided rope, etc. One especiallypreferred material is compressed particles of expanded (or exfoliated)graphite, sometimes referred to as flexible graphite. Especially usefulare sheets of compressed particles of exfoliated graphite.

The graphite useful in forming the joint strengthening rings of thepresent invention is a crystalline form of carbon comprising atomscovalently bonded in flat layered planes with weaker bonds between theplanes. By treating particles of graphite, such as natural graphiteflake, with an intercalant of, e.g. a solution of sulfuric and nitricacid, the crystal structure of the graphite reacts to form a compound ofgraphite and the intercalant. The treated particles of graphite arehereafter referred to as “particles of intercalated graphite.” Uponexposure to high temperature, the intercalant within the graphitevolatilizes, causing the particles of intercalated graphite to expand indimension as much as about 80 or more times its original volume in anaccordion-like fashion in the “c” direction, i.e. in the directionperpendicular to the crystalline planes of the graphite. The exfoliatedgraphite particles are vermiform in appearance, and are thereforecommonly referred to as worms. The worms may be compressed together intoflexible sheets that, unlike the original graphite flakes, can be formedand cut into various shapes.

Graphite starting materials for the sheets suitable for use in thepresent invention include highly graphitic carbonaceous materialscapable of intercalating organic and inorganic acids as well as halogensand then expanding when exposed to heat. These highly graphiticcarbonaceous materials most preferably have a degree of graphitizationof about 1.0. As used in this disclosure, the term “degree ofgraphitization” refers to the value g according to the formula:

$g = \frac{3.45 - {d(002)}}{0.095}$where d(002) is the spacing between the graphitic layers of the carbonsin the crystal structure measured in Angstrom units. The spacing dbetween graphite layers is measured by standard X-ray diffractiontechniques. The positions of diffraction peaks corresponding to the(002), (004) and (006) Miller Indices are measured, and standardleast-squares techniques are employed to derive spacing which minimizesthe total error for all of these peaks. Examples of highly graphiticcarbonaceous materials include natural graphites from various sources,as well as other carbonaceous materials such as carbons prepared bychemical vapor deposition and the like. Natural graphite is mostpreferred.

The graphite starting materials for the sheets used in the presentinvention may contain non-carbon components so long as the crystalstructure of the starting materials maintains the required degree ofgraphitization and they are capable of exfoliation. Generally, anycarbon-containing material, the crystal structure of which possesses therequired degree of graphitization and which can be exfoliated, issuitable for use with the present invention. Such graphite preferablyhas an ash content of less than twenty weight percent. More preferably,the graphite employed for the present invention will have a purity of atleast about 94%. In the most preferred embodiment, the graphite employedwill have a purity of at least about 99%.

A common method for manufacturing graphite sheet is described by Shaneet al. in U.S. Pat. No. 3,404,061, the disclosure of which isincorporated herein by reference. In the typical practice of the Shaneet al. method, natural graphite flakes are intercalated by dispersingthe flakes in a solution containing e.g., a mixture of nitric andsulfuric acid, advantageously at a level of about 20 to about 300 partsby weight of intercalant solution per 100 parts by weight of graphiteflakes (pph). The intercalation solution contains oxidizing and otherintercalating agents known in the art. Examples include those containingoxidizing agents and oxidizing mixtures, such as solutions containingnitric acid, potassium chlorate, chromic acid, potassium permanganate,potassium chromate, potassium dichromate, perchloric acid, and the like,or mixtures, such as for example, concentrated nitric acid and chlorate,chromic acid and phosphoric acid, sulfuric acid and nitric acid, ormixtures of a strong organic acid, e.g. trifluoroacetic acid, and astrong oxidizing agent soluble in the organic acid. Alternatively, anelectric potential can be used to bring about oxidation of the graphite.Chemical species that can be introduced into the graphite crystal usingelectrolytic oxidation include sulfuric acid as well as other acids.

In a preferred embodiment, the intercalating agent is a solution of amixture of sulfuric acid, or sulfuric acid and phosphoric acid, and anoxidizing agent, i.e. nitric acid, perchloric acid, chromic acid,potassium permanganate, hydrogen peroxide, iodic or periodic acids, orthe like. Although less preferred, the intercalation solution maycontain metal halides such as ferric chloride, and ferric chloride mixedwith a sulfuric acid, or a halide, such as bromine as a solution ofbromine and sulfuric acid or bromine in an organic solvent.

The quantity of intercalation solution may range from about 20 to about150 pph and more typically about 50 to about 120 pph. After the flakesare intercalated, any excess solution is drained from the flakes and theflakes are water-washed. Alternatively, the quantity of theintercalation solution may be limited to between about 10 and about 50pph, which permits the washing step to be eliminated as taught anddescribed in U.S. Pat. No. 4,895,713, the disclosure of which is alsoherein incorporated by reference.

The particles of graphite flake treated with intercalation solution canoptionally be contacted, e.g. by blending, with a reducing organic agentselected from alcohols, sugars, aldehydes and esters which are reactivewith the surface film of oxidizing intercalating solution attemperatures in the range of 25° C. and 125° C. Suitable specificorganic agents include hexadecanol, octadecanol, 1-octanol, 2-octanol,decylalcohol, 1, 10 decanediol, decylaldehyde, 1-propanol, 1,3propanediol, ethyleneglycol, polypropylene glycol, dextrose, fructose,lactose, sucrose, potato starch, ethylene glycol monostearate,diethylene glycol dibenzoate, propylene glycol monostearate, glycerolmonostearate, dimethyl oxylate, diethyl oxylate, methyl formate, ethylformate, ascorbic acid and lignin-derived compounds, such as sodiumlignosulfate. The amount of organic reducing agent is suitably fromabout 0.5 to 4% by weight of the particles of graphite flake.

The use of an expansion aid applied prior to, during or immediatelyafter intercalation can also provide improvements. Among theseimprovements can be reduced exfoliation temperature and increasedexpanded volume (also referred to as “worm volume”). An expansion aid inthis context will advantageously be an organic material sufficientlysoluble in the intercalation solution to achieve an improvement inexpansion. More narrowly, organic materials of this type that containcarbon, hydrogen and oxygen, preferably exclusively, may be employed.Carboxylic acids have been found especially effective. A suitablecarboxylic acid useful as the expansion aid can be selected fromaromatic, aliphatic or cycloaliphatic, straight chain or branched chain,saturated and unsaturated monocarboxylic acids, dicarboxylic acids andpolycarboxylic acids which have at least 1 carbon atom, and preferablyup to about 15 carbon atoms, which is soluble in the intercalationsolution in amounts effective to provide a measurable improvement of oneor more aspects of exfoliation. Suitable organic solvents can beemployed to improve solubility of an organic expansion aid in theintercalation solution.

Representative examples of saturated aliphatic carboxylic acids areacids such as those of the formula H(CH₂)_(n)COOH wherein n is a numberof from 0 to about 5, including formic, acetic, propionic, butyric,pentanoic, hexanoic, and the like. In place of the carboxylic acids, theanhydrides or reactive carboxylic acid derivatives such as alkyl esterscan also be employed. Representative of alkyl esters are methyl formateand ethyl formate. Sulfuric acid, nitric acid and other known aqueousintercalants have the ability to decompose formic acid, ultimately towater and carbon dioxide. Because of this, formic acid and othersensitive expansion aids are advantageously contacted with the graphiteflake prior to immersion of the flake in aqueous intercalant.Representative of dicarboxylic acids are aliphatic dicarboxylic acidshaving 2-12 carbon atoms, in particular oxalic acid, fumaric acid,malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid,1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid,1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid andaromatic dicarboxylic acids such as phthalic acid or terephthalic acid.Representative of alkyl esters are dimethyl oxylate and diethyl oxylate.Representative of cycloaliphatic acids is cyclohexane carboxylic acidand of aromatic carboxylic acids are benzoic acid, naphthoic acid,anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- andp-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoicacids and, acetamidobenzoic acids, phenylacetic acid and naphthoicacids. Representative of hydroxy aromatic acids are hydroxybenzoic acid,3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid,4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid,5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and7-hydroxy-2-naphthoic acid. Prominent among the polycarboxylic acids iscitric acid.

The intercalation solution will be aqueous and will preferably containan amount of expansion aid of from about 1 to 10%, the amount beingeffective to enhance exfoliation. In the embodiment wherein theexpansion aid is contacted with the graphite flake prior to or afterimmersing in the aqueous intercalation solution, the expansion aid canbe admixed with the graphite by suitable means, such as a V-blender,typically in an amount of from about 0.2% to about 10% by weight of thegraphite flake.

After intercalating the graphite flake, and following the blending ofthe intercalant coated intercalated graphite flake with the organicreducing agent, the blend is exposed to temperatures in the range of 25°to 125° C. to promote reaction of the reducing agent and intercalantcoating. The heating period is up to about 20 hours, with shorterheating periods, e.g., at least about 10 minutes, for highertemperatures in the above-noted range. Times of one-half hour or less,e.g., on the order of 10 to 25 minutes, can be employed at the highertemperatures.

The thus treated particles of graphite are sometimes referred to as“particles of intercalated graphite.” Upon exposure to high temperature,e.g. temperatures of at least about 160° C. and especially about 700° C.to 1200° C. and higher, the particles of intercalated graphite expand asmuch as about 80 to 1000 or more times their original volume in anaccordion-like fashion in the c-direction, i.e. in the directionperpendicular to the crystalline planes of the constituent graphiteparticles. The expanded, i.e. exfoliated, graphite particles arevermiform in appearance, and are therefore commonly referred to asworms. The worms may be compressed together into flexible sheets that,unlike the original graphite flakes, can be formed and cut into variousshapes and provided with small transverse openings by deformingmechanical impact as hereinafter described.

Flexible graphite sheet and foil are coherent, with good handlingstrength, and are suitably compressed, e.g. by roll-pressing, to athickness of about 0.075 mm to 3.75 mm and a typical density of about0.1 to 1.5 grams per cubic centimeter (g/cc). From about 1.5-30% byweight of ceramic additives can be blended with the intercalatedgraphite flakes as described in U.S. Pat. No. 5,902,762 (which isincorporated herein by reference) to provide enhanced resin impregnationin the final flexible graphite product. The additives include ceramicfiber particles having a length of about 0.15 to 1.5 millimeters. Thewidth of the particles is suitably from about 0.04 to 0.004 mm. Theceramic fiber particles are non-reactive and non-adhering to graphiteand are stable at temperatures up to about 1100° C., preferably about1400° C. or higher. Suitable ceramic fiber particles are formed ofmacerated quartz glass fibers, carbon and graphite fibers, zirconia,boron nitride, silicon carbide and magnesia fibers, naturally occurringmineral fibers such as calcium metasilicate fibers, calcium aluminumsilicate fibers, aluminum oxide fibers and the like.

The flexible graphite sheet can also, at times, be advantageouslytreated with resin and the absorbed resin, after curing, enhances themoisture resistance and handling strength, i.e. stiffness, of theflexible graphite sheet as well as “fixing” the morphology of the sheet.Suitable resin content is preferably at least about 5% by weight, morepreferably about 10 to 35% by weight, and suitably up to about 60% byweight. Resins found especially useful in the practice of the presentinvention include acrylic-, epoxy- and phenolic-based resin systems,fluoro-based polymers, or mixtures thereof. Suitable epoxy resin systemsinclude those based on diglycidyl ether or bisphenol A (DGEBA) and othermultifunctional resin systems; phenolic resins that can be employedinclude resole and novolac phenolics. Optionally, the flexible graphitemay be impregnated with fibers and/or salts in addition to the resin orin place of the resin.

The flexible graphite sheet material exhibits an appreciable degree ofanisotropy due to the alignment of graphite particles parallel to themajor opposed, parallel surfaces of the sheet, with the degree ofanisotropy increasing upon roll pressing of the sheet material toincreased density. In roll pressed anisotropic sheet material, thethickness, i.e. the direction perpendicular to the opposed, parallelsheet surfaces comprises the “c” direction and the directions rangingalong the length and width, i.e. along or parallel to the opposed, majorsurfaces comprises the “a” directions and the thermal and electricalproperties of the sheet are very different, by orders of magnitude, forthe “c” and “a” directions.

The thusly-formed flexible graphite sheet, formed so as to have therequired central opening can be used as is, or it can be formed into alaminate of several such flexible graphite sheets (without or without aninterlayer adhesive) and used as the inventive joint strengthening ringin that manner. Most preferably, though, because of the anisotropicnature of sheets of compressed particles of expanded graphite, theorientation of the graphite sheet joint strengthening ring should besuch that the “a” direction, that is the direction parallel to the majoropposed surfaces of the sheet, is directionally arrayed between the endfaces of the electrodes. In this way, the higher electrical conductivityof the material in the “a” direction will improve the conductivityacross the joint, as opposed to the “c” direction.

One embodiment of the inventive joint strengthening ring is illustratedin FIG. 1 and designated by the reference character 10. Jointstrengthening ring 10 comprises a spiral wound sheet of flexiblegraphite, and has its “a” direction through the thickness of jointstrengthening ring 10, rather than along its surface. Jointstrengthening ring 10 can be formed, for instance, by winding one ormore flexible graphite sheets around a bolster 100 having a diameterequal to the desired diameter of the central opening “d” of jointstrengthening ring 10. The sheets are wound around bolster 100 until aradius equal to the desired radius of joint strengthening ring 10 isachieved, resulting in a spiral wound flexible graphite cylinder 20,which can be sliced into individual joint strengthening rings 10 of thedesired thickness (either through bolster 100 or after removal ofbolster 100). In this way, the “a” direction of higher conductivity isarrayed through the thickness of joint strengthening ring 10.Optionally, an adhesive can be interposed between the windings of jointstrengthening ring 10 in order to prevent the spiral-wound jointstrengthening ring 10 from unwinding.

Alternatively, joint strengthening ring 10 can be formed by winding oneor more flexible graphite sheets around a bolster 100 until a radiusequal to the desired radius of joint strengthening ring 10 is achieved,and spiral wound cylinder 20 then compressed into the final desiredthickness and shape. Indeed, as discussed above, the compression processcan be used to mold (e.g., by die molding or the like) a concave orcorrugated shape into joint strengthening ring 10, as illustrated inFIGS. 6 and 7, respectively, having arms 10 a and 10 b or ridges 10 cwhich abut electrodes 30 and/or 40. These shapes can provide evengreater compressibility to joint strengthening ring 10.

Joint strengthening ring 10 is positioned between the end faces ofadjoining graphite electrodes forming an electrode joint. For example,as illustrated in FIG. 3, when a graphite electrode 30 having a machinedmale threaded tang 32 is employed, joint strengthening ring 10 can beplaced on end face 34 of electrode 30 about tang 32. When electrode 30is then mated with an adjoining electrode having a machined femalesocket (not shown), therefore, joint strengthening ring 10 is positionedbetween the end faces of the adjoining electrodes. The same holds truefor electrode 40, illustrated in FIG. 4, which uses a pin 42 rather thana tang.

Advantageously, joint strengthening ring 10 is positioned on electrode30 during preparation of electrode 30, either at the forming plant or atthe furnace site but prior to being brought into position above thefurnace for loading onto the electrode column to reduce the operationalsteps of forming the joint (which often takes place in a relativelyhazardous environment). Likewise, when pin 42 is pre-set into electrode40, joint strengthening ring 10 can be positioned on electrode 40 at thesame time. Moreover, when joint strengthening ring 10 is formed in aconcave shape, as shown in FIG. 6, and the concave portioned filled witha paste or cement, etc., a release liner can be used to protect thepaste or cement from dirt, dust, or other undesired substances whichmight otherwise adhere to it.

Accordingly, in use, electrode end-face joint strengthening ring 10 ispositioned between the adjoining electrodes 50 a and 50 b in anelectrode joint 50, as illustrated in FIG. 5. The compressible nature ofjoint strengthening ring 10 increases the high column bending strengthof a column formed using electrodes 50 a and 50 b by permittingcompression during bending, thus reducing the tendency of one or both ofelectrodes 50 a and 50 b to crack, split, etc. In addition, since jointstrengthening ring 10 advantageously oxidizes at a rate equal to orslower than that of electrodes 50 a and 50 b, it reduces oxygen ingressinto joint 50 between the end faces of electrodes 50 a and 50 b andthereby reduces or eliminates oxidation of the threaded portions or pin32 or male tang 42, and/or other surfaces of joint 50, extending thelife and functionality of joint 50.

The disclosures of all cited patents and publications referred to inthis application are incorporated herein by reference.

The above description is intended to enable the person skilled in theart to practice the invention. It is not intended to detail all of thepossible variations and modifications that will become apparent to theskilled worker upon reading the description. It is intended, however,that all such modifications and variations be included within the scopeof the invention that is defined by the following claims. The claims areintended to cover the indicated elements and steps in any arrangement orsequence that is effective to meet the objectives intended for theinvention, unless the context specifically indicates the contrary.

1. A process for preparing a joint strengthening ring for use in anelectrode joint, the process comprising providing a sheet of compressedparticles of exfoliated graphite and winding the sheet to form a spiralwound joint strengthening ring suitable for use between the electrodesin an electrode joint, wherein a surface of the joint strengthening ringhas a concave cross-section.
 2. The process of claim 1 wherein the jointstrengthening ring has an outer diameter generally equal to the outerdiameter of the electrode joint and a central opening.
 3. The process ofclaim 2 wherein an adhesive is interposed between the layers of thespiral wound sheet of compressed particles of exfoliated graphite. 4.The process of claim 2 wherein the sheet of compressed particles ofexfoliated graphite is wound around a bolster having a diameter equal tothe central opening of the joint strengthening ring.
 5. The process ofclaim 4 wherein the sheet of compressed particles of exfoliated graphitewound around a bolster is cut to a desired thickness after winding. 6.The process of claim 1 wherein a surface of the joint strengthening ringhas a corrugated cross-section.
 7. A process for preparing a jointstrengthening ring for use in an electrode joint, the process comprisingproviding a sheet of compressed particles of exfoliated graphite andwinding the sheet to form a spiral wound joint strengthening ringsuitable for use between the electrodes in an electrode joint, wherein asurface of the joint strengthening ring has a corrugated cross-section.8. The process of claim 7 wherein the joint strengthening ring has anouter diameter generally equal to the outer diameter of the electrodejoint and a central opening.
 9. The process of claim 8 wherein anadhesive is interposed between the layers of the spiral wound sheet ofcompressed particles of exfoliated graphite.
 10. The process of claim 8wherein the sheet of compressed particles of exfoliated graphite iswound around a bolster having a diameter equal to the central opening ofthe joint strengthening ring.
 11. The process of claim 10 wherein thesheet of compressed particles of exfoliated graphite wound around abolster is cut to a desired thickness after winding.
 12. The process ofclaim 7 wherein a surface of the joint strengthening ring has a concavecross-section.